1. Field of the Invention
This invention relates generally to safety systems, such as supplemental restraint systems for vehicles, and more particularly, to a fault-tolerant circuit that will deliver at least a predetermined minimum triggering energy to the squib of a supplemental restraint system, or other safety device, notwithstanding the presence of plural faults, such as short-circuits.
2. Description of the Related Art
Supplemental restraint systems which typically are used in vehicles deploy an inflatable airbag that is coupled to a gas generating device. When a fault condition of the type requiring deployment of an inflatable airbag is present, an electronic control unit, in response to a fault condition signal, causes a predeterminable quantum of electrical energy to be delivered to an ignitable airbag squib. The squib, which is in communication with the gas generator, causes the generator to inflate the airbag by releasing rapidly an inert, non-toxic gas, such as nitrogen. The rate of release of the gas is sufficient to inflate the airbag generally within milliseconds of the onset of the fault condition.
Typically, a fault condition will result from a collision by the vehicle. A sensor, that may take the form of a normally-open switch, operates in response to an inertial mass to complete an electrical circuit that causes the electronic control unit to issue a relatively large current to the ignitable squib. Ultimately, this causes the gas generator to be activated, or ignited, resulting in the rapid release of the inert gas.
One disadvantage of the known inflatable airbag control arrangements is that a highly reliable, low resistivity, interconnection must be maintained between the electronic control unit and the ignitable squib. Such an interconnection cannot tolerate short-circuiting to ground or the application of battery potential on either cable that couples the control circuit to the airbag squib.
There is, therefore, the need for an arrangement that can withstand short-circuiting to ground, or to the vehicle's supply potential without unintentionally firing, or being precluded from firing in the event of an emergency condition. There is a need that such reliability be present even when one of the terminals of the airbag squib is itself short-circuited.
It is, therefore, an object of this invention to provide a system for firing a safety device wherein electrical components that are critical to the operation of the system can readily be tested for functionality.
It is another object of this invention to provide a system for firing a safety device wherein the number of electrical components required to achieve reliable system operation and testing is minimized.
It is also an object of this invention to provide a system for firing a safety device wherein the possibility of unintentionally firing the safety device is reduced, particularly including during testing of the system.
It is a further object of this invention to provide a system for firing an electrical safety device of the type fired by conduction of a current therethrough of at least predetermined magnitude, wherein the unintentional application of a supply potential directly to the safety device, such as would result from certain short-circuit fault conditions, will not result in the firing of the electrical safety device.
It is additionally an object of this invention to provide a system for firing selectable ones of a plurality of electrical safety devices wherein the number of possible fault conditions that might cause unintentional firing of the safety device is reduced.
It is yet a further object of this invention to provide a system for firing selectable ones of a plurality of electrical safety devices wherein individual ones of the electrical safety devices can be subjected to a diagnostic procedure.
It is also another object of this invention to provide a system for firing selectable ones of a plurality of electrical safety devices wherein a firing current is easily distributed and its magnitude controlled.
It is yet an additional object of this invention to provide a system for firing an electrical safety device wherein the number of possible fault conditions that might cause unintentional firing of the safety device is reduced.
It is still another object of this invention to provide a system for firing an electrical safety device wherein immunity to electromagnetic interference is increased.
It is a yet further object of this invention to provide a system for firing an electrical safety device wherein firing reliability is enhanced while simultaneously affording the ability to abort a firing process in the event it is determined, between the time subsequent to issuance of a firing command and prior to actual firing, that the firing command was issued in response to a fault condition.
It is also a further object of this invention to provide a system for firing selectable ones of a plurality of electrical safety devices which might be subject to plural simultaneous fault conditions, while retaining high system reliability, at reasonable cost.